Better error message when using `~` in statics

``` rust
static x: ~[int] = ~[123, 456];

fn main() {}
```

```
foo.rs:1:19: 1:30 error: constant contains unimplemented expression type
foo.rs:1 static x: ~[int] = ~[123, 456];
                            ^~~~~~~~~~~
error: aborting due to previous error
```

While not inaccurate, it should explain that `~` is not supported in statics and won't be, not that it's just unimplemented.
